In industrial situations some metals, e.g., titanium, aluminum, steel alloys and stainless steel, are shot-peened in order to increase the residual stress in the metal object. As a non-destructive test procedure, eddy current measurements are employed to determine whether the entire surface of the metal object was shot peened and whether the residual stress has the correct distribution with respect to the depth in the metal object. In other instances eddy currents are used to determine surface and sub-surface cracks or flaws in metal objects. Such defects produce relatively large changes in the induced currents and common eddy current equipment easily detects these changes. The SmartEddy.TM. system, manufactured by SES Corporation, Menlo Park, Calif., uses a two resistor, two coil bridge to detect amplitude and phase shifts produced by treated metal objects compared to a test standard. U.S. Pat. No. 5,610,515 modified the SmartEddy.TM. system to improve its sensitivity in determining residual stress in treated non-ferromagnetic metal objects by reducing the sensitivity of the apparatus to vibration and temperature changes.
Current induced into a metal object by an external coil is called eddy current. The use of eddy current technique to characterize the properties of metal objects is based on the relationship between a conductive metal object structure and electromagnetic properties. More specifically, eddy current testing relies on electromagnetic interaction between the coil driven by an alternating electrical current and the metal object under test. In order to measure applied or residual stresses with a conventional eddy current approach, the application of stress must change the electrical conductivity such that a detectable change in the test coil impedance will occur.
Presently, current commercial eddy current equipment is not sensitive enough to detect extremely small changes in conductivity in a treated metal object. Nor is present commercial eddy current equipment sensitive enough to detect changes in conductivity at the near-surface of a treated metal object, i.e., within 0.020 inch.
It is an object of the present invention to provide a method and equipment to detect very small changes in conductivity between a treated metal object and an untreated metal object sample used as a reference.
It is another object of the present invention to be operated at a wide range of frequencies thereby checking the sample at various depths.
It is another object of the present invention to be operated at high frequency in order to measure conductivity at the near-surface of a treated metal object.
It is a further object of the present invention to determine the residual stress of a metal object that has been subject to machining, shot-peening or other chemical and mechanical treatments.
It is a still further object of the present invention to avoid ambiguous results due to mechanical vibration and lift-off as the test coil is brought close to the sample.
These and further important objects of the present invention will become more apparent upon considering the following detailed description of the present invention.